The invention relates to improved electrical bushings with resin casting. In particular the invention relates to advances in the use of cast resin systems which enables improved fixing and sealing of an electrical bushing insulator to a transformer case.
Typically a grid of power lines distributes electricity from either the point of generation or substations throughout both metropolitan and rural areas. Power is normally distributed at 11-36KV from the point of generation or from substations. In order to effectively supply power (electricity) for domestic use in home or business, transformers are employed at substations so as to step down the voltage to lower levels for subsequent conducting through a distribution network. Such transformers typically supply domestic users with between 110 to 450 V of electricity. Therefore transformers are subject to high input voltages and lower output voltages which supply a subsequent power distribution network.
The transformer requires insulative conductors on both the high and low voltage sides to protect the transformer from shorting to the Earth. The transformer case is usually metal and must be safely insulated. Electrical bushings provide a safe means of passing electrical current through the transformer casing. Normally an electrical conductor is sealed within such an electrical bushing so as to prevent leakage between the bushing and the conductor and to present electrical terminals extending from the interior of the transformer tank.
Electrical bushings of the prior art are produced in a variety of configurations so that they can be permanently affixed upon an electrical transformer by a welding process or the like. The bushing presents an electrical conductor through an opening in the wall of a transformer into the interior thereof which is filled with transformer oil or inert gas. Conventional bushings are provided with an insulative component and an exterior mounting flange affixed upon the component, which flange is adapted to be welded upon the transformer tank in order to permanently fix the bushing to the transformer in a way which seals the opening therein. It is a requirement that the bushing provides a mechanical seal, air to air, air to gas, or air to oil, dependent on the transformer design.
In the prior art, bushings welded upon a transformer suffer the disadvantage of sacrificing the ability of the bushing or transformer to be easily repaired or replaced. It is a frequent occurrence, due to accident or malfunction, that the bushing requires repair or replacement, however because the bushing can""t be easily removed without extensive labour, complete replacement of the transformer is often the end result.
Further disadvantages of the prior art such as oil leakage, may occur because the welding process to seal the bushing into the transformer may not form a complete seal, it is often the case that such leakage will not be detected until final stages of manufacture when corrective measures are costly.
Generally the prior art can be regarded in two categoriesxe2x80x94ceramic and non-ceramic. Ceramic prior art has been limited in design due to the materials employed. For example porcelain bushings cannot be cast in asymmetrical shapes. Also, the difference in expansion coefficient between the conductor and the insulator means that the conductor can never be cast into the bushing. This limitation complicates both the assembly of the unit as a whole, the fixing of the bushing to the transformer and also the sealing of the bushing unit.
It is desirable for a bushing to be able to be held in angular relation to a transformer casing. This angular configuration is usually achieved by providing a modified transformer casing with a gusset upon which a porcelain bushing is seated. Transformer casings which are modified to include a gusset are expensive and time consuming to construct. Prior art porcelain bushings and assemblies have so far been unable to meet this desire.
Non Ceramic prior art addresses some of these shortcomings, for example the casting of the conductor into the resin system, but does not adequately address the fixing of the bushing to the transformer and sealing of the bushing to the transformer case. Prior art has also explored various metallic clamping devices on the inside or outside of the transformer, all of which detract from the insulative capacity of the bushing.
Against the foregoing background, it is an object of the present invention to overcome the disadvantages of the prior art.
It is a further object of the invention to provide an improved bushing which advances the use of cast resin systems for a bushing design.
A further object of the invention is to provide an improved bushing assembly which greatly improves and facilitates the fixing and sealing of the bushing to a transformer case. In particular the new improved bushing serves to effectively provide a mechanical seal thereby preventing any leakage, but at the same time enables easy removal of the bushing so that a transformer doesn""t have to be fully replaced.
The invention in a first aspect relates to an improved cast resin bushing assembly for use in a transformer wherein the bushing assembly comprises:
a bushing which bushing includes locating means for aligning the bushing assembly or part thereof within an opening in a transformer casing;
wherein the bushing further includes a locking means for releasably securing the bushing assembly to a transformer casing;
a bushing conductor integrally moulded to said bushing; and
a bushing flange in circumferential arrangement with the bushing.
In a second aspect the invention relates to a method of securing an improved cast resin bushing assembly to a transformer casing which bushing assembly comprises:
a bushing which bushing includes a locating means for aligning the bushing assembly or part thereof within an opening in a transformer casing;
the bushing further includes a locking means for releasably securing the bushing assembly to a transformer casing;
a bushing conductor integrally moulded to said bushing; and
a bushing flange in circumferential arrangement to the bushing;
positioning said locating means on said bushing within said transformer opening so as to align said bushing assembly;
mounting said bushing flange on an outside surface surrounding the transformer opening;
releasably securing the bushing assembly within the transformer casing by engaging said locking means inside the transformer casing.
Preferably the locking means is a threaded section moulded into the bushing adapted to be engaged by a corresponding collar inside a transformer casing.
Preferably the threaded section and the collar are cast in the same mould.
Preferably the collar can be a cast resin nut.
Preferably the locating means comprises a groove or flat portion for aligning the bushing assembly within an opening in a transformer casing.
Preferably the bushing flange is adapted to be mounted on a transformer casing.
Preferably the bushing flange has up to four locating holes for additional securement to a transformer casing.
Preferably the bushing conductor is cast within the bushing.
Preferably the bushing assembly is cast in a one step mould process.
In one embodiment of the invention there is disclosed a method of releasably securing an improved cast resin bushing assembly to a transformer casing by providing a bushing assembly including:
a bushing which bushing includes a groove or flat portion for aligning the bushing assembly or part thereof within an opening in the transformer casing;
the bushing further includes a threaded section for securing the bushing assembly to the transformer casing;
a threaded collar;
a bushing conductor integrally moulded to said bushing; and
a bushing flange in circumferential arrangement with the bushing;
positioning said locating means on said bushing within said transformer opening so as to align said bushing assembly;
mounting said bushing flange on an outside surface surrounding the transformer opening;
releasably securing the bushing assembly within the transformer casing by engaging the threaded section on the bushing on the inside of the transformer casing with the threaded collar.
In a further specific embodiment it is preferred that the collar is a cyclo nut. In yet a further preferred embodiment the bushing flange includes locking holes which can be used as an additional or alternate means of securing the bushing assembly to the transformer casing whereby studs or nuts are lockably engaged to secure the bushing flange against the transformer casing.
Preferably the cast resin system is selected from Epoxy, Phenolic, Polyester, Vinyl Ester, Polyurethane and related composites. More preferably the cast resin system is a cycloaliphatic Epoxy Resin System.
Preferably the bushing assembly is cast in asymmetric or symmetric design.
Preferably the improved bushing assembly further comprises a flange moulded to the bushing wherein the flange is substantially annular and has an outside diameter larger than the transformer opening.
Preferably the threaded section of the bushing is cast in a resin moulding together with a complementary nut or collar.
In use customers have a choice as to which arrangement best suits their purpose. In some instances for example in subterranean applications the threaded cast resin section and nut or collar are preferred because the use of studs in this application may be a point of potential corrosion initiation.